Pension and Other Postretirement Benefits - Schedule of Weighted Average Assumptions Used to Determine Benefit Obligations and Net Periodic Benefit Cost (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase	3.28%	3.61%
Expected long-term rate of return on plan assets	6.89%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.68%	4.12%
Rate of compensation increase	3.28%
Discount rate	3.61%	3.45%	4.09%
Expected long-term rate of return on plan assets	7.47%	7.31%	6.91%
Rate of compensation increase	3.61%	3.65%	3.74%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.54%	3.93%
Discount rate	3.30%	2.90%	4.15%